1. Field of the Invention
This invention relates to a hydraulic control system for pressurizing clutches in an automatic transmission. The invention pertains more particularly to a hydraulic control system for an automatic transmission which, upon sensing an unacceptable combination of pressurized clutches, reverts to the combination that produces a predetermined speed ratio.
2. Description of the Prior Art
An automatic transmission for automotive use includes planetary gearsets whose elements are hydraulically connected or held in order to produce selected ratios of the engine crankshaft speed to the speed of the output of the transmission. Automatic gear change transmissions may be controlled by electronic means which produce analog current signals that operate solenoid valves. These valves connect hydraulic line pressure to selected clutches and brakes, or vent the hydraulic cylinder of these elements to atmosphere. The control system should operate such that, in the event of a control system malfunction, the transmission may operate in at least one forward speed ratio and in reverse drive in order to allow the vehicle to be driven to a service station for repair.
Line pressure in an automatic transmission is usually established by connecting the output of a hydraulic pump to a pressure regulator valve through which the regulated fluid is directed to selectively operated shift valves, which control the engagement of clutches and brake servos whose selective engagement produces the various speed ratios of which the transmission is capable. If the pressure applied to the various clutches differs beyond a certain limit or if the clutches that are concurrently pressurized is inconsistent with the schedule or pressurized clutches required to produce the several drive ratios of the transmission, then incorrect operation will result. It is preferred, in the event of such unscheduled clutch engagement or malfunction of the electronic control that operates solenoid valves, that the hydraulic control system automatically operate to engage those clutches required to produce a predetermined forward speed ratio. Furthermore, reverse drive operation should result without reference to the electronic control.